Design of Rectangular Pile Cap

INTRODUCTION TO THE SPREADSHEET FOR DESIGN OF RECTANGULAR PILE CAP:

In this section, the general features of the spreadsheet are explained briefly. This spreadsheet can be used for the design (as per BS 8110-1:1997) of rectangular pile caps with 4 or more piles.

1.0 ASSUMPTIONS & LIMITATIONS OF THE SPREADSHEET:

1) The pile cap is assumed to be rigid.
2) This spreadsheet is meant only for rectangular pile caps. Octagonal or irregular shaped pile caps are outside the purview of this spreadsheet.
3) This spreadsheet is meant only for pile caps with single pedestal/column. Pile caps for multi-pedestals/columns are outside the purview of this spreadsheet.
4) Minimum number of piles shall be 4. (Separate spreadsheets are made for pile caps with 2 piles & 3 piles). Maximum number of piles shall be 28.
5) Centroid of pedestal is considered as the origin for co-ordinate axes.
6) Eccentricity of horizontal loads with respect to C.G. of pile cap is not considered.
7) Maximum load cases is limited to 100.
8) The diagrams showing the plan of pile cap is indicative only; meant to show the direction of co-oridnate axes. The number of piles/size of pile cap shown
will not change based on the input.
9) For checking the buoyancy condition, only two conditions are considered – design water table either at ground level or below the founding depth of pile cap.
Intermediate levels between these two are not considered.

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2.0 FEATURES OF THE SPREADSHEET:

1) STAAD analysis output can be directly fed as the input for design of pile cap.
2) There are options to design the pile cap either for the actual pile loads or for the full capacity of pile. In the case of design with full capacity of piles, all the
pile loads will be taken as compression capacity for calculating the bottom reinforcement and all the pile loads will be taken as tension capacity for calculating the top reinforcement.
3) For easy reference, all inputs for the spreadsheet are highlighted in “Rose” colour and the output are highlighted in “Yellow” colour.
4) All cells other than the input cells are locked to protect the spreadsheet from any unintended change/modification.
5) Effect of water table is considered in the spreadsheet. However, the loss in self weight of pile due to water table shall be accounted in the tension capacity of pile.

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3.0 PROCEDURE FOR USING THE SPREADSHEET:

The procedure for using the spreadsheet is briefly explained below.
“Input” Sheet:

1) The output (unfactored support reactions) from STAAD for the required pedestal/column along with the load case number shall be pasted under “Loads from STAAD Output”. Care shall be taken to ensure that the units are in “kN” & “m” and proper direction of co-ordinate axes is selected.
2) Select “Yes” or “No” for “Whether load at bottom of pile cap is available or not?”
3) Enter the number of load cases in the cell for “Total number of load cases”.
Take care to give the exact number of load cases, because otherwise the output will give misleading values.

“Design” Sheet:

1) Under “General Data” provide various input like size of pile, number of piles, depth of water table, capacity of piles, grade of material, dimensions of pile cap, founding depth of pile cap, clear cover to reinforcement, dimensions of pedestal, etc. in the proper units in the respective cells.
2) Under “Arrangement of Piles”, furnish the co-ordinates of each pile in metres, based on the direction of co-ordinate axes, as shown in the diagram of pile cap.
3) Enter the location of critical sections for shear in mm, with respect to the CG of pile cap.
4) Under “Design of Pile Cap”, select the option for design with “Actual Pile Reactions” or “Full Capacity of Piles”, as the case be.
5) Select the diameter of reinforcement to be used, suiting the conditions.
6) Look for various checks (failure messages) and complete the design.
7) Remember to click the “Perform Design” button each time the design input is changed.
8) All the calculations are performed in the sheet “Calculation”, which is protected. However, the relevant output is written to sheets “Pile Loads” and “Critical BM & SF”, which are unprotected to enable submission of output.

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